I think you'll find the M-Audio USB FastTrack Pro to be a good match for what you want; cheaper than FireWire, and I believe that it uses the built-in OS X USB Class drivers, which means that there are NO drivers for you to install--it just works right out of the box.
I had a FireWire Solo, and its another inexpensive, solid solution you might want to consider.
but if you want something waaaaaaay easier, yet runs on your laptop's power with no need for a wall wart, try a Tascam US122. great little device that i use w/ my mobile laptop / gig recording rig. you only get 2 simultaneous inputs, but hey, that's the price you pay.
It may be worth noting the Fast Track USB model doesn't supply phantom power for condenser mic's. The Pro model of course does. You probably already know this but I've seen a few people buy one without knowing. So I thought it worth mentioning.
The Presonus Inspire is a Firewire unit that is pretty much the same price as the M-Audio Fast Track Pro and can also run bussed powered if you want to get a FW interface. Although you may prefer the I/O on something else. And the Presonus Firebox is also within your budget.